XF 2.1 How to remove the usergroup permission check for user upgrade payment checkout?

[RobinHood]

I'm trying to make a site where you have to pay to become a member and view any content.

As such, you can register, but the registered usergroup is locked down by setting their view permission to 'No'.

The user account settings are still accessible though, as is the User Upgrades page, which is great.

But when I then try to purchase a user upgrade, I'm denied access to the Stripe checkout modal.

How can I remove or edit the permission check for this modal so that valid, registered users can successfully purchase an upgrade to gain access to the rest of the site?

I've looked in templates:

payment_initiate_stripe
account_upgrades

But I can't see a permission check, can anyone advise how I can work around this?

Thanks